The charge will simply be the access fee you pay when you arrive, and yes, spectators are welcomed.
Things will be marked off as to where you can and can't go. If in doubt, just ask. Be respectful, and it will be reciprocated.
I flew out in '93 to my first LSR event at El Mirage - it changed my life. I built a car, raced at Maxton and Bonneville, held a Bonneville record for 7 years, visited El Mirage twice since, including this last May, and have made some of the closest friendships I've ever known.
Keep an eye on SCTA's facebook page, though - you'll want to confirm that the event is on.
Welcome aboard!